All possible kinds of relationship for Hibernate mapping. Combined into a table format for easier referencing. Taken from Xylax.net

Relationship	Bidirectionality
Simple Relationship (one-to-one) Java Code `Bar Foo.getBar() // returns corresponding Bar instance` Hibernate Mapping `<class name="Foo" table="foo"> ... <one-to-one name="bar" class="Bar"/> </class>`	Yes. Just add a similar mapping and `Foo` property to `Bar`
Simple Reference (many-to-one) Java Code `Bar Foo.getBar() // returns corresponding Bar instance` Hibernate Mapping `<class name="Foo" table="foo"> ... <many-to-one name="bar" class="Bar" column="bar_id"/> </class>`	Yes. Just add a similar mapping and `Foo` property to `Bar`. Just add an extra column in Bar's table called `foo_id`
Basic Collection (one-to-many) Java Code `Set Foo.getBars() // returns a collection of Bar instances` Hibernate Mapping `<class name="Foo" table="foo"> ... <set role="bars" table="bar"> <key column="foo_id"/> <one-to-many class="Bar"/> </set> </class>`	Relationship can be established both ways. Just refer to the many-to-one relationship
Collection (many-to-many) Java Code `Set Foo.getBars() // returns a collection of Bar instances` Hibernate Mapping `<class name="Foo" table="foo"> ... <set role="bars" table="foo_bar"> <key column="foo_id"/> <many-to-many column="bar_id" class="Bar"/> </set> </class>`	Relationship can be established both ways. Just refer to the many-to-one relationship

Glossary:
First-rank classes are those with their own mapping definition for Hibernate entities and must always be explicitly persisten.
Second-rank classes are those with enclosed by first-rank classes or persisted throught associations or relationships with a first-rank class.

Source:
Xylax.net

Simple Relationship (one-to-one)

Simple Reference (many-to-one)

Basic Collection (one-to-many)

Collection (many-to-many)